In an electronic apparatus, a laser printer for example, a paper feed roller (driven component) disposed in a main body case is connected to a motor, and the paper feed roller is rotated by driving the motor to feed the paper to a specified portion.
The motor generally used is a brushless DC motor. The motor comprises a stator having a plurality of magnetic poles arranged circumferentially at first spaced intervals along the outer periphery, and a rotor arranged along the outer periphery of the stator. And, structurally, magnets magnetized in a heteropolar fashion are circumferentially arranged at second spaced intervals along the inner periphery of the rotor.
Also, the magnetic pole of the stator is formed with an extended portion extended in a direction nearly parallel to a permanent magnet from the pole base portion, thereby enhancing the driving efficiency.
That is, the width (perpendicular to circumferential direction) of the permanent magnet is greater than the width (perpendicular to circumferential direction) of the pole base portion of the stator so that the rotor rotation is as proximate as possible to the magnetic detector element which magnetically detects the rotor rotation. And, an extended portion extended in a direction nearly parallel to the permanent magnet is formed from the pole base portion of the stator. In this way, the opposed area between the magnetic pole of the stator and the magnet is increased in order to enhance the driving force and driving efficiency. Similar technologies are disclosed for example in Unexamined Japanese Patent Publication H9-285044 (Patent document 1) and Unexamined Japanese Patent Publication 2007-244004 (Patent document 2).
As described above, in the case of a conventional motor wherein an extended portion extended in a direction substantially parallel to the permanent magnet is formed from the pole base portion of the magnetic pole of the stator, the opposed area between the permanent magnet of the rotor and the magnetic pole of the stator becomes larger. Accordingly, it has been generally considered that the driving force and driving efficiency can be enhanced.
However, according to the examination made by the present inventor, it has been found that it is not always possible to enhance the driving efficiency by only disposing an extended portion.
That is, in the case of a conventional motor formed with an extended portion as described above, the magnetic flux from the opposed permanent magnet increases with increase in elongation of the extended portion. In that case, magnetic saturation becomes liable to take place due to increase of the magnetic flux. As a result, there arises a problem that the driving force and driving efficiency cannot be enhanced.